Pick any key to see its relative minor, the closely related keys either side of it on the circle, and the seven diatonic chords built from that scale. Everything runs offline in the browser.
Moving clockwise round the circle adds a sharp; anticlockwise adds a flat. Neighbouring keys share six of their seven notes, which is why they modulate smoothly. Diatonic chords in a major key follow the pattern I ii iii IV V vi vii°.